Sonar systems use transducers to transmit and receive sound signals. Arrays of hydrophones are often arranged on the surface of the hull of a vessel (e.g. a submarine or surface ship) and placed to detect sound waves omni-directionally about the vessel. Sonar hull arrays are being developed with an emphasis towards lightweight, conformal, “Paste-On” solutions which may be attached to portions of the hull structure. The design of these next-generation arrays include accelerometers having increased vertical aperture size and particle velocity sensors.
Present accelerometer designs include configurations using fiber optics or flexural piezoelectric disks. Fiber optic solutions tend to be expensive and complicated. As a result, piezoelectric options are often considered more cost effective. Piezoelectric transducers operate to generate electricity when subjected to a pressure change. Accordingly, piezoelectric materials embodied in piezo transducers convert a sound signal (i.e. pressure wave traveling through water) into an electrical signal. Piezoelectric transducers have an impedance which closely matches that of water, making them highly effective when in contact with water as the transmitting medium. Piezoelectric transducers are less effective in air, or in an underwater housing sealed from the water, due at least in part to the impedance mismatch between the piezoelectric material and air.
Referring now to FIG. 1 there is shown a conventional piezoelectric transducer accelerometer 100. The accelerometer 100 includes an outer housing 101 integrated with a base 102. A piezoelectric element 105 is fixedly attached at one end to base 102. The opposite end of piezoelectric element 105 is attached to a high density proof mass 103. The accelerometer 100 is neutrally buoyant, including the housing 101 which moves back and forth as sound waves pass over the accelerometer 100. As the outer housing 101 moves with the sound waves, the high density proof mass 103 resists the movement, thereby causing a relative motion between the outer housing 101 and the high density proof mass 103. The relative motion between the outer housing 101 and the high density proof mass 103 is transmitted through the piezoelectric material in the piezoelectric element 105, causing a flexing stress on the piezoelectric element 105. This in turn generates an electrical voltage representative of the movement caused by the sound wave.
FIG. 2 shows a piezoelectric transducer 200 based on a flexural piezoelectric disk 207. A neutrally buoyant outer layer 201 encloses a housing 203. A substrate 204 is disposed within the housing 203 and is in contact with the walls of the housing 203. A piezoelectric disk 207 is attached to each side of the substrate 204 using an adhesive. When the piezoelectric transducer 200 is in contact with a passing sound wave, the housing 203 is moved by the passing sound wave. The piezoelectric disks 207, which also act as the proof mass (103 shown in FIG. 1) resist the movement of the housing 203 and cause the substrate 204 to flex. The piezoelectric disks 207 are adhered to the substrate and conformally flex along with the substrate 204.
The flexural piezoelectric design shown in FIG. 2 presents certain design challenges. The flexural piezoelectric transducer 200 has limited bandwidth, is subject to delamination and damage to piezoelectric disk 207, and is costly to manufacture. Alternative designs that provide higher bandwidth and sensitivity and that are easier and less expensive to produce are desired.